Transmitting an optical signal with corresponding WDM and TDM channels

ABSTRACT

An optical signal transmitter is disclosed that transmits an optical signal with corresponding Wavelength Division Multiplexing (WDM) channels and Time Division Multiplexing (TDM) channels. The transmitter is comprised of a laser, a dispersion system, and a modulator. The laser generates and transmits a narrow laser pulse comprised of a plurality of wavelength channels. The dispersion system broadens the narrow laser pulse into a wide laser pulse. The modulator modulates the wide laser pulse based on an electric modulation signal comprised of a plurality of time slot channels wherein the time slot channels in the electric modulation signal correspond to the wavelength channels in the wide laser pulse respectively. The modulator transfers a modulated wide laser pulse. Channels of the modulated wide laser pulse are hybrid wavelength and time slot channels. The transmitter singularly transmits a WDM optical signal comprised of multiple wavelength channels. The transmitter also allows a filter to drop time slot channels from a TDM optical signal instead of an expensive demultiplexer.

RELATED APPLICATIONS

This application is a continuation of prior application Ser. No. 09/556,232, filed on Apr. 24, 2000 now U.S. Pat. No. 6,587,238, and that is herein incorporated by reference into this application.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable

MICROFICHE APPENDIX

Not applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the field of optical communication systems, and in particular, a transmitter configured to transmit an optical signal with corresponding Wavelength Division Multiplexing (WDM) and Time Division Multiplexing (TDM) channels.

2. Description of the Prior Art

In a fiber optic system, a transmitter transmits an optical signal over a fiber to a receiver. Designers of fiber optic systems work to increase transfer capacities of the system to accommodate more data traffic and the Internet. Transmitters use Wavelength Division Multiplexing (WDM) and Time Division Multiplexing (TDM) to increase the fiber optic system capacities.

A conventional transmitter is comprised of a laser and a modulator. The laser generates a laser signal and transfers the laser signal to the modulator. The modulator modulates the laser signal to represent data to be transmitted over an optical fiber. The modulator modulates either directly or externally. To directly modulate, the modulator varies an injected current supplied to the laser and changes the intensity of the laser signal. To externally modulate, the modulator receives the laser signal and an electric modulation signal. The electric modulation signal represents the data to be transmitted over the fiber. The modulator changes the amplitude and/or phase of the laser signal based on the electric modulation signal. An example of an external modulator is a Mach-Zehnder interferometer.

Time Division Multiplexing (TDM) transmits a number of signals simultaneously over a communications medium by interleaving sections of the individual data signals one after another. TDM divides a bandwidth of a TDM optical signal into fixed time slots of a fixed capacity, wherein the time slots represent time slot channels. To implement TDM in a fiber optic system, a time division multiplexer receives the individual data signals and assigns a time slot channel to the individual data signals. The time division multiplexer breaks the data signals into sections. The time division multiplexer plugs the sections of the data signals into the time slots of the TDM optical signal corresponding to the time slot channel assigned to the data signals.

In a TDM fiber optic system, nodes in the system are assigned time slot channels. Nodes drop the assigned time slot channels from the TDM optical signal using a demultiplexer, such as a Synchronous Optical NETwork (SONET) Add-Drop Multiplexer (ADM).

A problem with TDM is the fiber optic system uses multiple demultiplexers to drop the time slot channels from the TDM optical signal at the nodes. The separate nodes use separate demultiplexers to drop the time slot channels from the TDM optical signal. The number of demultiplexers depends on the number of nodes in the system. Demultiplexers are relatively expensive and increase the cost of the TDM fiber optic system.

Wavelength Division Multiplexing (WDM) passes multiple data channels over one or more wavelengths of light simultaneously over a single fiber. To implement WDM in a fiber optic system, multiple transmitters transmit optical signals over separate wavelength channels to a multiplexer. The multiplexer combines the optical signals to produce a WDM optical signal comprised of multiple wavelength channels. For example, a first transmitter transmits a first optical signal over a first wavelength channel, a second transmitter transmits a second optical signal over a second wavelength channel, and a third transmitter transmits a third optical signal over a third wavelength channel. A multiplexer receives and multiplexes the first, second, and third optical signals to produce the WDM optical signal comprised of a first wavelength channel, a second wavelength channel, and a third wavelength channel.

In a WDM fiber optic system, nodes in the system are assigned wavelength channels. Nodes drop the assigned wavelength channels from the WDM optical signal using an Optical Add-Drop Multiplexer (O-ADM). One example of the O-ADM is a conventional filter. Nodes also add the wavelength channels back to the WDM optical signal.

A problem with WDM is the fiber optic system uses multiple transmitters to transmit the WDM optical signal. WDM uses separate transmitters for the separate wavelength channels in the WDM optical signal. As the number of wavelength channels in the WDM optical signal increase, the number of transmitters increases raising the cost of the fiber optic system.

SUMMARY OF THE INVENTION

An optical signal transmitter transmits an optical signal with corresponding Wavelength Division Multiplexing (WDM) and Time Division Multiplexing (TDM) channels to solve the above problems. The transmitter is comprised of a laser, a dispersion system, and a modulator. The laser transmits a narrow laser pulse comprised of a plurality of wavelength channels. The dispersion system broadens the narrow laser pulse into a wide laser pulse. The modulator modulates the wide laser pulse based on an electric modulation signal comprised of a plurality of time slot channels wherein the plurality of time slot channels in the electric modulation signal correspond to the plurality of wavelength channels in the wide laser pulse respectively. The channels of the modulated wide laser pulse are hybrid wavelength and time slot channels.

The transmitter advantageously reduces the cost of a fiber optic system. The transmitter can be used at a central node so that multiple channels can share a common laser and modulator. WDM typically requires a separate transmitter for each channel of a WDM optical signal. The transmitter in this embodiment of the invention singularly generates a WDM optical signal with multiple channels. Similarly, TDM typically requires expensive demultiplexers to drop channels at respective nodes in the fiber optic system. With the hybrid wavelength and time slot channels of the invention, nodes in the fiber optic system can drop a TDM channel from a WDM and TDM optical signal with a filter as is commonly done in a WDM system.

In another embodiment of the invention, the transmitter further comprises a control system configured to generate the electric modulation signal. The control system is comprised of a delay system and a multiplexer. The delay system receives a plurality of data signals and delays the data signals individually by a different multiple of a specific time period. The multiplexer receives and multiplexes the delay data signals to generate the electric modulation signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram depicting a transmitter in an example of the invention.

FIG. 2 is a block diagram depicting a fiber optic system in an example of the invention.

FIG. 3 is a block diagram depicting a transmitter including a control system in an example of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Transmitter Operation and Configuration—FIGS. 1–2

FIG. 1 depicts a specific example of a transmitter 110 in accord with the present invention. Transmitter 110 is comprised of a laser 120, a dispersion system 122, and a modulator 124. Laser 120 is coupled to dispersion system 122. Dispersion system 122 is coupled to modulator 124. Laser 120 is configured to generate and transmit a narrow laser pulse 126. Dispersion system 122 is configured to broaden the narrow laser pulse 126 into a wide laser pulse 128. Modulator 124 is configured to receive the wide laser pulse 128 and an electric modulation signal 132, modulate the wide laser pulse 128 based on the electric modulation signal 132, and transfer a modulated wide laser pulse 130.

In operation, laser 120 generates the narrow laser pulse 126 comprised of wavelength channels λ₁, λ₂, . . . λ_(n) and transmits the narrow laser pulse 126 to dispersion system 122. The wavelength channels λ₁, λ₂, . . . , λ_(n) are specific wavelength bands typical of WDM. The narrow laser pulse 126 is any laser pulse between approximately 50 fs –100 fs. Dispersion system 122 broadens the narrow laser pulse 126 into the wide laser pulse 128. The wide laser pulse 128 is a laser pulse greater than approximately 100 fs. The wide laser pulse 128 is comprised substantially of the wavelength channels λ₁, λ₂, . . . , λ_(n). Dispersion system 122 transfers the wide laser pulse 128 to modulator 124. Modulator 124 receives the wide laser pulse 128 and the electric modulation signal 132. The electric modulation signal 132 represents data to be transmitted by transmitter 110. The electric modulation signal 132 is comprised of time slot channels 1, 2, . . . , n. The time slot channels 1, 2, . . . , n are groups of specific time slots typical of TDM. Modulator 124 modulates the wide laser pulse 128 based on the electric modulation signal 132 wherein the wavelength channels λ₁, λ₂, . . . , λ_(n) in the wide laser pulse 128 correspond to the time slot channels 1, 2, . . . , n in the electric modulation signal 132 respectively. After modulation, channels 1, 2, . . . , n of the modulated wide laser pulse 130 are hybrid wavelength and time slot channels. Modulator 124 transfers the modulated wide laser pulse 130 more descriptively referred to as a WDM and TDM optical signal 130.

FIG. 2 shows transmitter 110 implemented in a fiber optic system 200. One example of fiber optic system 200 is as a Metropolitan Area Network (MAN). Fiber optic system 200 is comprised of a central node 234 and nodes 241–243. Central node 234 is comprised of transmitter 110. Nodes 241–243 are comprised of filters 251–253 respectively. Transmitter 110 and nodes 241–243 are coupled in series by a fiber 236. In operation, transmitter 110 transmits the WDM and TDM optical signal 130 over fiber 236. Node 241 receives the WDM and TDM optical signal 130. Filter 251 drops a first channel from the WDM and TDM optical signal 130 by filtering the wavelength channel 8, from the WDM and TDM optical signal 130. The first channel is a hybrid wavelength and time slot channel. By filtering the wavelength channel λ₁, filter 251 automatically drops the time slot channel 1. Therefore, filter 251 performs TDM demultiplexing in this embodiment. Node 241 filters out the wavelength channel 8 ₁ without substantially affecting neighboring channels. Node 241 transfers the WDM and TDM optical signal 130 to node 242.

Node 242 receives the WDM and TDM optical signal 130. Filter 252 drops a second channel from the WDM and TDM optical signal 130 by filtering the wavelength channel λ₂ from the WDM and TDM optical signal 130. The second channel is a hybrid wavelength and time slot channel. By filtering the wavelength channel 2, node 242 automatically drops the time slot channel 2. Filter 252 filters out the wavelength channel λ₂ without substantially affecting neighboring channels. Node 242 transfers the WDM and TDM optical signal 130 to node 243. The same operation takes place in node 243. Fiber optic system 200 could alternatively be comprised of n nodes.

Transmitter 110 advantageously transmits the WDM and TDM optical signal 130 with corresponding WDM and TDM channels. Transmitter 110 can be used at a central node so that multiple channels share a common laser 120 and modulator 124. Transmitter 110 singularly transmits a WDM optical signal with multiple channels instead of multiple transmitters. Transmitter 110 also allows nodes to drop time slot channels from a TDM optical signal with a simple filter instead of an expensive demultiplexer. Transmitter 110 provides for a cheaper fiber optic system 200 for implementing WDM and/or TDM.

Transmitter with a Delay System and Multiplexer Configuration and Operation—FIG. 3

FIG. 3 depicts a specific example of a transmitter 310 in accord with the present invention. Those skilled in the art will appreciate numerous variations from this example that do not depart from the scope of the invention. Those skilled in the art will also appreciate that various features could be combined to form multiple variations of the invention.

Transmitter 310 is comprised of a laser 320, a dispersion system 322, a modulator 324, and a control system 340. Control system 340 is comprised of a delay system 342 and a multiplexer 344. Laser 320 is coupled to dispersion system 322. Dispersion system 322 is coupled to modulator 324. Modulator 324 is coupled to multiplexer 344. Multiplexer 344 is coupled to delay system 342.

In operation, laser 320 generates a narrow laser pulse 326 comprised of wavelength channels λ₁, λ₂, . . . , λ_(n) and transfers the narrow laser pulse 326 to dispersion system 322. The wavelength channels λ₁, λ₂, . . . , λ_(n) are specific wavelength bands typical of WDM. Laser 320 is a Femto second laser with a repetition rate of approximately 150 Mbit/s, but alternatively the repetition rate could vary depending on requirements. The narrow laser pulse 326 is any laser pulse between approximately 50 fs–100 fs. The range of the wavelength channels λ₁, λ₂, . . . , λ_(n) is between approximately 30 nm–40 nm. Dispersion system 322 broadens the narrow laser pulse 326 into a wide laser pulse 328. The wide laser pulse 328 can also be referred to as a chirped broad pulse. Dispersion system 322 is a short section of fiber with high dispersion. High dispersion is dispersion greater than normally occurring in a single mode fiber. For example, a high dispersion fiber is a Dispersion Compensation Fiber (DCF) with a dispersion of −80 ps/km-nm. The high dispersion properties of the short section of fiber spreads the narrow laser pulse 326 into the wide laser pulse 328. Dispersion system 322 could generate negative dispersion to have an opposite dispersion effect as a standard transmission fiber. The wide laser pulse 328 is a laser pulse greater than approximately 100 fs. The wide laser pulse 328 is comprised substantially of the wavelength channels λ₁, λ₂, . . . , λ_(n). Dispersion system 322 transfers the wide laser pulse 328 to modulator 324.

Concurrently with laser 320, delay system 342 receives n data signals 334 from a plurality of sources (not shown). The data signals 334 are STS-3 signals in a Return-to-Zero (RZ) format. Delay system 342 delays the individual data signals 334 by a different multiple of a specific time period T. For instance, delay system 342 delays a first data signal by 1*T, a second data signal by 2*T, and an nth data signal by n*T. Delay system 342 generates T-sized time slots by delaying the data signals 334 by multiples of T. The time slots represent time slot channels 1, 2, . . . , n. The time slot channels 1, 2, . . . , n are groups of specific time slots typical of TDM. Delay system 342 transfers delayed data signals 336 to multiplexer 344. Multiplexer 344 multiplexes the delayed data signals 336 to generate an electric modulation signal 332. The electric modulation signal 332 is comprised of the T-sized time slots representing the time slot channels 1, 2, . . . , n. The electric modulation signal 332 is a multiplexed STS-3 signal.

Modulator 324 receives the wide laser pulse 328 and the electric modulation signal 332. Modulator 324 is an Electro-Absorption (EA) modulator or a LiNbO3 modulator. Modulator 324 modulates the wide laser signal 328 based on the electric modulation signal 332 wherein the wavelength channels λ₁, λ₂, . . . , λ_(n) in the wide laser pulse 328 correspond to the time slot channels 1, 2, . . . , n in the electric modulation signal 332 respectively. More particularly, the time slot channels 1, 2, . . . , n have a central wavelength of λ₁, λ₂, . . . , λ_(n) respectively. After modulation, the channels 1, 2, . . . , n of a modulated wide laser pulse 330 are hybrid wavelength and time slot channels. For instance, modulator 324 modulates the first data signal at the wavelength channel λ₁ and the time slot channel 1. Modulator 324 modulates the second data signal at the wavelength channel λ₂ and the time slot channel 2. Modulator 324 modulates the nth data signal at the wavelength channel λ_(n) and the time slot channel n.

Modulator 324 transfers the modulated wide laser pulse 330 more descriptively referred to as a WDM and TDM optical signal 330. The WDM and TDM optical signal 330 is an OC-3 signal. A channel bit rate of the WDM and TDM optical signal 330 should be substantially the same as a repetition rate of laser 320, which is approximately 150 Mbit/s.

With transmitter 310 implemented in a fiber optic system, a node in the fiber optic system that drops the wavelength channel λ₁ from the WDM and TDM optical signal 330 automatically drops the time slot channel 1 because the time slot channel 1 has a central wavelength of wavelength λ₁. A filter within the node filters out the wavelength channel λ₁ without substantially affecting neighboring channels. The filter replaces a relatively expensive demultiplexer to drop the time slot channel 1.

The nodes in the fiber optic system also add channels back to the WDM and TDM optical signal 330. The channel bit rate is not limited to OC-3 and can be either RZ or Non-Return-to-Zero (NRZ) format.

Transmitter 310 advantageously transmits the WDM and TDM optical signal 330 with corresponding WDM and TDM channels. Transmitter 310 can be used at a central node so that multiple channels can share a common laser 320 and modulator 324. Transmitter 310 singularly transmits a WDM optical signal with multiple channels instead of multiple transmitters. Transmitter 310 also allows nodes to drop time slot channels from a TDM optical signal with a simple filter instead of an expensive demultiplexer. Transmitter 310 provides for a cheaper fiber optic system for implementing WDM and/or TDM.

Those skilled in the art will appreciate variations of the above-described embodiments that fall within the scope of the invention. As a result, the invention is not limited to the specific examples and illustrations discussed above, but only by the following claims and their equivalents. 

1. An optical communication system comprising: an optical fiber; a transmitter system configured to transmit an Optical Carrier-3 (OC-3) optical signal on the optical fiber, wherein the OC-3 optical signal is comprised of a plurality of wavelengths and a first plurality of time slots and represents a plurality of Synchronous Transport Signal-3 (STS-3) channels comprising timeslots, wherein each time slot of each of the STS-3 channels corresponds to a wavelength of the plurality of wavelengths and a time slot of the first plurality of time slots; a first node configured to receive the optical signal over the optical fiber and to divert a first wavelength from the optical signal to divert a first STS-3 channel that corresponds to the first wavelength and pass the remaining wavelengths of the plurality of wavelengths; and a second node coupled in series to the first node by the optical fiber and configured to receive the optical signal over the optical fiber from the first node and divert a second wavelength from the optical signal to divert a second STS-3 channel that corresponds to the second wavelength.
 2. The optical communication system of claim 1 wherein the transmitter system is configured to generate the OC-3 optical signal and to modulate the OC-3 optical signal based on an electric modulation signal comprised of the plurality of STS-3 channels.
 3. The optical communication system of claim 2 wherein the OC-3 optical signal comprises a laser pulse.
 4. The optical communication system of claim 3 wherein the transmitter system further comprises a dispersion system configured to broaden the laser pulse into a wide laser pulse.
 5. The optical communication system of claim 2 wherein the transmitter system comprises a modulator system configured to modulate the OC-3 optical signal with the electric modulation signal.
 6. The optical communication system of claim 5 wherein the transmitter system further comprises a multiplexer coupled to the modulator system and configured to receive delayed data signals, multiplex the delayed data signals to generate the electric modulation signal, and transfer the electric modulation signal to the modulator system.
 7. The optical communication system of claim 6 wherein the transmitter system further comprises a delay system coupled to the multiplexer and configured to receive a plurality of data signals, delay the data signals individually by a different multiple of a time period, and transfer the delayed data signals to the multiplexer.
 8. The optical communication system of claim 7 wherein the plurality of data signals are in Return-to-Zero (RZ) format.
 9. The optical communication system in claim 1 wherein the first node is further configured to add the first wavelength back to the OC-3 optical signal.
 10. The optical communication system in claim 1 wherein the second node is further configured to add the second wavelength back to the OC-3 optical signal.
 11. A method of filtering an Optical Carrier-3 (OC-3) optical signal comprising: transmitting the OC-3 optical signal on an optical fiber wherein the OC-3 optical signal is comprised of a plurality of wavelengths and a first plurality of time slots and represents a plurality of Synchronous Transport Signal-3 (STS-3) channels comprising timeslots wherein each time slot of each of the STS-3 channels corresponds to a wavelength of the plurality of wavelengths and a time slot of the first plurality of time slots; receiving the OC-3 optical signal into a first node; diverting a first wavelength from the OC-3 optical signal that corresponds to a first STS-3 channel and passing the remaining wavelengths of the plurality of wavelengths from the first node to a second node coupled in series to the first node; receiving the OC-3 optical signal into the second node from the first node; and diverting a second wavelength from the OC-3 optical signal that corresponds to a second STS-3 channel.
 12. The method of claim 11 further comprising generating the OC-3 optical signal and modulating the OC-3 optical signal based on an electric modulation signal comprised of the plurality of STS-3 channels.
 13. The method of claim 12 wherein the OC-3 optical signal comprises a laser pulse.
 14. The method of claim 13 further comprising broadening the laser pulse into a wide laser pulse.
 15. The method of claim 12 further comprising multiplexing a plurality of data signals to produce the electric modulation signal.
 16. The method of claim 15 further comprising delaying the data signals individually by a different multiple of a time period.
 17. The method of claim 11 comprising transmitting the OC-3 optical signal at a rate of approximately 150 Mbit/s. 